In the automotive field for example, improvement in collision safety is demanded together with improvement in fuel consumption by weight reduction of vehicle bodies for environmental conservation. Thus, use of high-strength steel plates for thickness reduction and optimization of vehicle body structures have been practiced hitherto in various ways, so as to achieve weight reduction of vehicle bodies and improvement in collision safety.
Fatigue strength is also demanded in the high-strength steel plates for weight reduction of vehicle bodies. In general, the fatigue strength of a parent material increases in proportion to steel plate strength, but it is known that the fatigue strength of a welded joint barely increases even when the steel plate strength is increased. This hinders the weight reduction of vehicle bodies by use of the high-strength steel plates.
In particular, underbody members such as suspension arms and sub-frames are difficult to be reduced in weight because the fatigue strength of welded portions will be a problem. In general, fillet arc welding is used for welding such underbody members, and thus increasing the fatigue strength of fillet arc welded joints will be an issue. Hereinafter, the “fillet arc welded joint” will be described as “fillet welded joint” as necessary.
To address such an issue, based on knowledge that the fatigue strength of the lap fillet welded joint is related to the radius of curvature of a toe portion and this radius of curvature depends on chemical components of welded metal, Patent Literature 1 discloses a method to increase the radius of curvature of a toe portion by optimizing chemical components of the welded metal, so as to reduce stress concentration and improve the fatigue strength.
However, the method disclosed in Patent Literature 1 does not have an effect to reduce stress concentration to a root portion. Further, when the stress concentration to the toe portion decreases, stress concentration to the root portion relatively becomes obvious, and a fatigue failure may occur with the root portion being a starting point.
Patent Literature 2 discloses a weld bead structure in which an end face of another steel plate is butted against one surface of a steel plate, a fillet bead is formed on both sides of the butted portion, and moreover, this weld bead is extended.
The weld bead structure disclosed in Patent Literature 2 is to alleviate stress concentration to a toe portion by keeping the toe portion away from the end portion of another steel plate. However, there is no effect to reduce stress concentration in the root portion. Moreover, in the lap fillet welded joint, there is almost no stress concentration reducing effect in the toe portion, and it is not possible to effectively suppress occurrence of fatigue failure.
Further, Patent Literature 3 suggests a technique to reduce residual stress and stress concentration in the welded portion and to increase the fatigue strength by performing fillet welding of a main plate and a rib plate in boxing welding of the rib plate, cooling them thereafter to room temperature, and disposing a linear weld on an end portion of the rib plate to be longer than the “rib plate thickness+2×fillet welding leg length” by a length equal to or more than “2×fillet welding leg length”.
However, the technique described in Patent Literature 3 is targeted at a steel material which is a thick plate of 15 mm to 25 mm, and cannot be applied to a welded portion of a thin steel plate of approximately 3.6 mm or less, which is used for underbody members or the like of automobiles. Specifically, in a T-shaped joint of thin plate, in view of welding efficiency, fillet welding from both sides of a vertical plate (plate corresponding to a rib plate) is rarely performed. Further, when welding of an end portion of a thin plate is performed like boxing, the end portion of the vertical plate welded portion melts down due to heat input at the time of welding, and an undercut defect occurs.
Furthermore, with respect to the residual stress in the welded portion which is the problem in Patent Literature 3, the residual stress in the welded portion increases by constraints of the parent material itself in a welded portion of the thick plate. Meanwhile, in welding of a thin plate, the residual stress is relatively small since the plate can easily deform out of plane. On the other hand, since out-of-plane deformation easily occurs in a thin-plate welded member, the welded portion is twisted when a tensile load is inputted, and stress concentration may increase not only in the weld toe portion but also in the root portion, which makes it necessary to consider a technique to suppress a crack occurring from the both.
As described above, in joint forms such as the lap fillet welded joint and the one-side fillet arc welded joint, if a load which can cause opening of the root portion is applied, the degree of concentration of stress may become larger in the root portion than in the toe portion. However, there has been proposed no technique to reduce the stress concentration to the root portion and effectively suppress occurrence of fatigue failure with the root portion being a starting point.